This invention relates to an apparatus for pulling a web of flexible bag-making material for a packaging machine of the form-fill-seal type. More particularly, the invention relates to such an apparatus with an improved mechanism for positioning pull-down belts and transmitting power to the belts for a form-fill-seal packaging machine of the vertical pillow type which does not require the presence of a loading cylinder of a conventional type.
Conventional form-fill-seal packaging machines of the so-called vertical pillow type are usually comprised of a shoulder-shaped member disposed above a vertical cylinder. A web of belt-like elongated flexible thermoplastic film material, from which bags are to be formed, is pulled from a film supply roll and is wrapped around this vertical cylinder by means of the shoulder-shaped member. For this reason, this shoulder-shaped member is sometimes referred to as a former. The side edges of the belt-like film material are overlapped and these overlapped parts are thermally sealed as the film material is pulled down along the outer surface of the cylinder such that the film material is made into a tubular form. This sealing process is hereinafter referred to as the vertical sealing. A horizontally elongated heater is provided below the cylinder for a horizontal sealing process whereby the film material now in a tubular form is sealed horizontally into a bag-like form. In the meantime, articles to be packaged are dropped into this newly formed bag through the vertical cylinder which may therefore be referred to also as a loading cylinder.
In order to form bags of uniform height, such a bag-making apparatus as described above is generally provided with a pair of so-called pull-down belts for pulling down the film material by a constant distance. Such pull-down belts are generally provided with air cylinders and mechanical handles such that their separation can be varied for changing the size of the bags or for setting a new web to the machine. With the apparatus formed as described above, however, it is difficult to adjust the pressure to be applied onto the film material.
When a relatively large amount of articles is dropped into each bag, the use of a loading cylinder may not present a serious problem. When only a small amount is to be packaged individually, however, the thickness of the side wall of the cylinder itself may have the effect of significantly reducing the cross-sectional area through which the articles to be packaged can flow into the bag and this, in turn, adversely affects the efficiency of the packaging machine as a whole.
Moreover, a packaging machine of this type is usually provided with a lever-like member inside the film material so as to keep the bag in open position. Should such a member break, broken pieces thereof would be likely to remain inside the bag with packaged articles. If the loading cylinder is dispensed with, however, there arises the problem of accurately positioning the bags.